1. Field of the Invention
This invention relates generally to the data switching and asynchronous multiplexing in a synchronous data communication system.
2. Discussion of the Background
It is well known that there is an ever increasing need for higher speeds within communication controllers for the transfer of data.
More specifically, increased and ever growing pressure for enhanced office efficiency require that increased data communication speeds be available on "voice/data systems". As these systems grow in cost and complexity, it is also desirable that they be readily and efficiently expanded, without the necessity of abandoning previously purchased components. A continuously expandable modularized approach to such systems is thus highly desirable.
Conventional time division multiplex systems are well known in the art, for example, as disclosed in U.S. Pat. Nos. 4,390,982 to Williams et al or 3,924,077 to Blakeslee, however, these are synchronous only type systems which waste communication capabilities. Specifically, in a pure synchronous time division multiplex system each "timeslot channel" which is carved out of the communication media by the time division multiplexing is reserved as a specific channel for a given communication path. This given communication path may not be shared by multiple devices in a logically simultaneous manner. Consequently the medium which is synchronously divided by the time division multiplexing is to a large extent wasted during idle periods between the different devices or endpoints coupled to the system. Additionally, automatic baud rate recognition and adaptation cannot be accomplished with these types of systems. Specifically, if two devices of different baud rates are attached to the system, the devices must be reset to operate at the same baud rate. However, not all devices are capable of changing their baud rate by way of a software command, which would be necessary for these types of systems to operate. Alternatively, these systems require that all devices connected to the communication bus operate at the same baud rate through some type of special controller card which does a hardware conversion of the incoming and outgoing bit streams to the appropriate hardware at each endpoint.
An alternative to synchronous systems are systems embodying asynchronous division of a communication medium or bus by way of various multiple access techniques, such as used in the Ethernet.TM. local area network system disclosed in U.S. Pat. No. 4,292,623 to Eswaran et al. The CSMA system disclosed in U.S. Pat. No. 4,542,499 to Bella et al is similar. However, the asynchronous use of communication media raises several problems. Further, a traditional CSMA/CD system such as is contemplated by the Ethernet.TM. local area network system is limited in the physical length to which the communication media may extend. Ethernet protocols specifically require that the system dimentions be kept under one kilometer. This one kilometer limitation applies to terminal to terminal communication distances, a serious limitation in the modern office environment.
Other prior art systems have used polling techniques in which a central controller sequentially polls each of the remote terminals or end points, offering each an opportunity to access the communication bus when available. In one such system, called a priority request system, remote end points ready to transmit a message make a request, and are granted access to the bus according to priorities established by a bus arbiter at the central controller. However, systems that use the priority request technique require that a significant amount of processing to be done by the central controller.
Other channel access systems such as shown in the Kume U.S. Pat. Nos. 4,538,261 and 4,506,361, use a retransmission mechanism based on a probing of the communication bus to determine if packets have collided on the bus. This technique results in inefficiencies in that individual packets may be retransmitted multiple times before a clear transmission of the packet can be accomplished.
Additionally, a need exists in the modern office environment for communication between terminals or endpoints in a system to access leased T1 channel or X.25 interface in the most expeditious and efficient manner. For these reasons there is at present a need for an improved, modularized data switching system.